This invention relates to electric power systems and, more particularly, to such systems which include circuits for detecting imminent failure of the system voltage.
DC link variable speed constant frequency (VSCF) electric power systems, such as those typically used in aircraft applications, include a generator which is driven at a variable speed to supply a DC voltage to an inverter by way of a pair of DC link conductors. The output voltage of the generator is controlled by a regulator which monitors the inverter output and adjusts the generator field current to regulate the DC link voltage. Other circuits use a similar arrangement in which a converter derives various output voltages from a DC input. In certain applications of these circuits, such as in flight critical power systems, there is a need to supply a warning that the power on the main output bus is corrupt or about to be corrupt due to a drop in the DC link voltage.
The design of a circuit for supplying such a warning is complicated by the fact that the input and output power conductors of the system typically have separate ground lines for safety reasons and to minimize electromagnetic interference. One known technique for supplying the necessary warning signal has been the use of a dedicated switching module which chops the DC link voltage and transformer couples it to a control circuit where it is peak detected. When the peak value falls below some preselected threshold, a warning signal is produced.
This approach suffers from the fact that it requires a separate module designed to operate off of a high voltage primary DC input. This adds to the size of the system and to the amount of noise generated. It also increases the complexity and decreases the mean time before failure of the system. In addition, if the module fails, no warning of line failure is given. Therefore, it is desirable to devise a power system having a circuit for detecting impending line failure that is simple, inexpensive and fault tolerant.